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# Who decides?

Year 9 Biology · Coordination and control

<!-- Speaker note: Students build stimulus–response pathways, name the CNS as the command centre with sensory and motor neurons, and explain why reflexes are fast (AC9S9U01, node 2, a hinge node). About 20 minutes (Engage and Explore 8 min · Explain 7 min · Evaluate 3 min · Typical adds 2 min, Wider about 2 min). -->

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## Today

**We are learning to** the central nervous system (brain and spinal cord) is the command centre: sensory neurons carry impulses in from receptors, motor neurons carry them out to effectors. Reflexes go through the spinal cord without waiting for the brain.

- I can build a stimulus–response pathway, choosing the receptor, command centre and effector.
- I can name sensory and motor neurons and the CNS.
- I can explain why a reflex is faster than a decided response.

<!-- Speaker note: Before class: A concept tutorial for introducing or revising the idea; it does not replace the ruler-drop practical. Set the width with `?width=minimum`, `typical` (default) or `wider`. The hinge misconception is that the brain decides every response. The drawing pin is a required pathway. Board notes: the menus mix receptors, command centres and effectors from all four stimuli; a wrong choice names the first step that is wrong. -->

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## Why it matters

> Step on a pin and your foot is off the floor before you've said 'ouch'. Your brain wasn't the one who decided.

<!-- Speaker note: Every response follows the same pathway. This lesson: building the pathway yourself, and which part of the nervous system decides. -->

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<span style="background:#dbeafe; color:#1a1a1a; border:1.5px solid #3b82f6; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">💬 Discussion</span>

## Engage: predict

**You step on a drawing pin and lift your foot instantly. Which part decided to lift it?**

- The foot itself
- The leg muscles
- The spinal cord
- The brain

<!-- Speaker note: What happens: The spinal cord. It's a reflex: the impulse goes from the receptor to the spinal cord and straight back out to the leg muscles. The brain finds out a moment later, which is when you feel the pain. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: count how many chose 'the brain'. -->

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<span style="background:#fef3c7; color:#1a1a1a; border:1.5px solid #f59e0b; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✍️ Activity</span>

## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fbiology-9%2Fpathways%2Fstep-2&code=widget%3A+coordination%0Amode%3A+build%0Aneed%3A+3%0Astimulus%3A+Stepping+on+a+drawing+pin+%7C+external+%7C+Pain+receptors+in+the+foot+%7C+Spinal+cord+%7C+Leg+muscles+%7C+You+lift+your+foot%0Astimulus%3A+A+ball+thrown+towards+you+%7C+external+%7C+Light+receptors+in+the+eyes+%7C+Brain+%7C+Arm+and+hand+muscles+%7C+You+catch+the+ball%0Astimulus%3A+The+smell+of+hot+chips+%7C+external+%7C+Smell+receptors+in+the+nose+%7C+Brain+%7C+Salivary+glands+%7C+Your+mouth+waters%0Astimulus%3A+Body+getting+too+cold+%7C+internal+%7C+Temperature+receptors+in+the+skin+and+brain+%7C+Hypothalamus+%28brain%29+%7C+Skeletal+muscles+%7C+You+shiver%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/biology-9/pathways/?full=step-2#step-2)

<!-- Speaker note: Pick a stimulus, then choose its receptor, command centre and effector, and send the signal. -->

<!-- Speaker note: Teacher: watch for students choosing the brain for the pin; let the board's message do the work. -->

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## What happened?

* The spinal cord. It's a reflex: the impulse goes from the receptor to the spinal cord and straight back out to the leg muscles. The brain finds out a moment later, which is when you feel the pain.

<!-- Speaker note: Reveal after running. Ask who predicted it, and what made them think so. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Neurons

**Match each part of the nervous system to its place in the pathway.**

| | |
|---|---|
| `Sensory neuron` | A. The command centre |
| `Brain and spinal cord (CNS)` | B. Turns a stimulus into an impulse |
| `Motor neuron` | C. Carries impulses from the CNS to an effector |
| `Receptor` | D. Carries impulses from a receptor to the CNS |

* Answer: Sensory neuron → D. Carries impulses from a receptor to the CNS; Brain and spinal cord (CNS) → A. The command centre; Motor neuron → C. Carries impulses from the CNS to an effector; Receptor → B. Turns a stimulus into an impulse

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<!-- Speaker note: Teacher: the reflex question is the checkpoint. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Reflexes

**Why is a reflex faster than catching a ball?**

- Pain impulses travel faster than light impulses
- Leg muscles are faster than arm muscles
- The impulse goes through the spinal cord only, with no time spent deciding in the brain
- Reflexes use hormones

* Answer: The impulse goes through the spinal cord only, with no time spent deciding in the brain

<!-- Speaker note:  -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Effectors

**In the chips pathway, the effector is a gland, not a muscle. What does an effector have to be?**

- Always a muscle
- A nerve
- A receptor
- A muscle or a gland: something that carries out the response

* Answer: A muscle or a gland: something that carries out the response

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## Elaborate: reflex or not?

**Reflex (automatic, no decision) or a decided response?**

Groups: Reflex · Decided response

Blinking when something flies at your eye · Answering a question in class · Pupils narrowing in bright light · Choosing a snack · Knee jerking when tapped below the kneecap · Catching a ball

* Answer: Blinking when something flies at your eye → Reflex; Answering a question in class → Decided response; Pupils narrowing in bright light → Reflex; Choosing a snack → Decided response; Knee jerking when tapped below the kneecap → Reflex; Catching a ball → Decided response

<!-- Speaker note: What this step is for: Sorting reflexes from decided responses uses the 'does it need the brain to decide?' test. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Elaborate: damage to the pathway

**A person's spinal cord is damaged in their upper back. A doctor taps below their knee. What would you expect?**

- Both work normally
- The knee can't jerk, but they feel the tap
- Nothing changes
- The knee-jerk may still work if the reflex arc below the damage is intact, but the person may not feel the tap

* Answer: The knee-jerk may still work if the reflex arc below the damage is intact, but the person may not feel the tap

<!-- Speaker note: Doctors use reflex tests to find where along the spinal cord damage is. What this step is for: Predicting the effect of a break at one point shows the pathway is a chain. -->

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## Evaluate: look back

Write one sentence: "At first I thought… Now I think… because…"

**Revision summary**

- The **central nervous system** (brain and spinal cord) is the command centre.
- **Sensory neurons** carry impulses from receptors to the CNS; **motor neurons** carry them to effectors (**muscles or glands**).
- A **reflex** is automatic and fast: the impulse goes through the spinal cord without waiting for the brain.

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Exit ticket

**Name the command centre in a reflex and say why reflexes are fast.**

* The spinal cord; the impulse doesn't have to travel to the brain and wait for a decision.

<!-- Speaker note: Students answer on paper or a mini whiteboard before the reveal. Anyone who cannot answer needs the main idea again next lesson. Where this leads: Next: nerves versus hormones (node 4), then feedback (node 5). In class: ruler-drop reaction times with and without distraction; knee-jerk reflex. -->